Experimental Comparison of Neutrino and Muon Velocities

The velocities of neutrinos and muons originating from ${\pi }^{+}$ and $K^{+}$ decays have been compared using the California Institute of Technology-Fermilab detector in a narrow-band neutrino beam. A time-of-flight comparison between positive muons and neutrinos, as indicated by the final-state negative muons in neutrino interactions, yields $|{\beta }_{\nu }-{\beta }_{\mu }|<~0.4\times {10}^{-3\mathrm{}}$ (99% confidence level). We assume that $|1-{\beta }_{\mu }|<\mathrm{}{10}^{-5\mathrm{}}$ for these muons.